Vacuum electric-tube device



Oct. 30, 1928. 1,689,971

E. Y. RoBmsoN VACUUM ELECTRIC TUBE DEVICE Filed June 27, 1924 2 ShgetS-Sheet 1 ZzweW 7 Wife)? Oct. 30, 1928. 1,689,971

E. Y. ROBINSON VACUUM ELECTRIC TUBE DEVICE Filed June 27, 1924 2 Sheets-Sheet 2 I'lil GO'O'IOI.

/ I 6 666 I Z flvzegi (r070 Patented a. 30, 1928.

UNITED STATES PATENT OFFICE.

ERNEST YEOMLN ROBINSON, WITHING'ION, HANCHESTEB, ENGLAND, ASSIGNOR TO IETBOPOLITAN-VICKERS ELECTRICAL COMPANY LIMITED, A GOKIAN'Y ORGAN- IZED UNDER LAWS OF GREAT BRITAIN.

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Application filed June 27,1924, Serial No.

This invention relates to vacuum electric tube devices of the kind employing heated filament cathodes such devices beingcommonly'termed thermionic valves or rectifiers.

Owing to the fact that the space current increases the actual filament current at the negative end of the filament and decreases it at the positive end a flow of space current in the device results in unequal heatingof the filament. The life of the filament and hence of the valve is dependenton the temperature of the hottest partofthe filament and since at the temperatures at which incandescent cathodes are operated, the rate of evaporation of the filament is extremely sensitive to the temperature of the filament it is necessary that the temperature should not be varled except within very narrow limits. For instance at the operating temperature of a tungsten filament in a thermionic valve a .3 per cent increase or decrease of filament current re--- spectively halves or doubles the life of the valve. In consequence of this it is-necessa ry to arrange in practice that the maximum emission current is not greater than 7 to 10 per cent of the filament current. In large valves this necessitates the use of high filament current which is undesirable.

In order to reduce this unequal heating of the filament alternating current can be used for heating the filament and the space current tapped to the mid point of the secondaryof the filament transformer or to the mid-point of a choke coil connected across the, filament terminals. It will be seen that bothin' this case and in the case wherein the filament is heated by direct current the space current is led into the filament at two points, namely at its two ends.

The object of the present invention is to decrease unequal heating of the filament due to the addition of the space current to the filament current proper or conversely the object of the invention is to increase the ratio of space current to filament current which may be used in the device to give the same degree of unequal heating of the filament and so permit the use of filament cathodes supplyin a large emission current but which do not ta e a large heating current. v

According to the invention means are pro vided whereby the space current is led into the filament at more than two points so that 722,810,- and in Great Britain any 54,

the unequal heating of the filament is reduced. Y

It is arranged that firstlythe filament is heated by alternating current, secondly the space current is led into thefilament at more than two points and thirdly an inductive winding or windings areconnected across the space current taps on the filament in such a manner that they prevent a short circuit of the filament portions while they present no appreciable impedance to the space current flowing tothe filamenttappings. The tap on .the' winding. is such that the potential thereof is the same as that of the corresponding filament tap; i

To enable-the invention to be clearly understood severalarrangements thereof will now be described with reference to the accompanying drawings in which Fig. 1 is an electrical diagram explaining the invention, Fig. 2 is a view of a'hairpin filament illustrating how the invention may be carried into effect according to one arrangement, Figs. 3 and 4 are front and side elevations respectively of a filament cathode 3. These electrodes are,

connected respectively to an external circuit which is indicated at 4 connected between conductors 5 and 6. The. filament cathode 3 is heated from an alternating current source (not shown) connected .to the primary wind ing of a transformer 7 the secondary winding of which is connected to the filament leads 8 and 9. The space current is supplied to the filament 3 by means of the leads 8 and 9 and also by means of a lead 10 which is connected to the mid-point 11 of the filament as shown, the said lead 10 being connectedto the midpoint 12 of a choke C011 13 which is connected across the filament leads 8 and 9, the conductor-6 being also connected to the mid point 12 of the choke coil and to the'mid-pointll of the filament. It will be seen that the actual potential of the mid-point 11 of the filament is equal to that of the mid-point 12 of the choke coil 13. The choke coil 13 prevents short-eircuiting of the filament 3 by the connections thereto for the space current.

Referring to Fig. 2 which illustrates one arrangement for carrying out the invention as illustrated in Fig. 1, the filament 3 of the hairpin type the ends of which are connected to supports which may constitute the lead-in conductors 8 and 9 and pass through a seal in the tube envelo e as will be well understood. The hairpin filament 3 is placed in tension by means of a spring 14 one end of which is attached at 15 to a member fixed in position with respect to the lead-in conductors 8 and 9 and the other end of which 15 provided with a hook 16 which engages the ight of the filament. The fixed end 15 of the spring 14 has connected to it the midpoint lead-in conductor 10 which also passes through the valve envelope so that it can be connected to the mid-point 12 of the choke coil 13. It will be understood that a similar effect can be obtained by omitting the coil 13 and connecting the conductors 6 and 10 to the mid-point of the secondary winding of the filament heating transformer 7.

Referring now to Figs. 3 and 4 the electrode structure therein illustrated comprises an anode 2, a twin filament cathode 3 constructed and mounted on a framework as described for example in the s ecification of my copending application erial, N0. 7 22,811, filed June 27, 1924. The framework comprises two parallel side members 16 of insu ating material such as pyrex lass connected together by means of a nic el member 17 at the upper end and a nickel member 18 at the lower end by which latter member the frame thus constituted with the electrodes mounted thereon is attached to a reentrant tube 19 of the tube envelope (not shown). The two parallel filaments 3 are at one end connected to two rigid supportin members 20 and 21 mounted upon the si e members 16 of the framework and insulated from one another. The members 20 and 21 have connected to them the lead-in wires 22 which ass through the seal 23 of the re-entrant tu e 19 and are connected with the external conductors 8 and 9.

The upper ends of the filaments are connected to a conducting stri or clamp 24 which in turn has connecte to it one end of the tensioning spring 14 the other end of which is attached to a hook 25 provided upon the frame member 17. The two filaments pass through perforations in a cross member 26 which consists of a metal strip of channel section having perforations at its ends through which the side members 16 of the framework pass. The strip 26 serves to register the filament in position with respect to the framework.

The anode 2 is constructed of two similar plates each having a pair of semi-circular channels extending longitudinally therein, the said plates being connected together so that two relatively small cylindrical tunnels are provided through which the filaments 3 pass. The anode thus constituted is supported from the framework by means of cross pieces 28 similar to the cross piece 26, one of which cross pieces 28, for example the lower one, is rigid, the anode being connected thereto by means of a conductor 29 welded both to said cross piece and to the anode.

With the arrangement above described it is possible to construct a valve of the kind set forth in the specification of Serial No. 603,715, filed November 27, 1922, that is to say, a valve in which the cathode is of a kind providing a concentrated source of electrons, the paths available for the space current are short, and the space current confined to the inter-electrode space, this last feature being obtained in the present example by maintaining the ends of the filaments 3 cool, namely, by means of the short-circuiting strip 27 secured across the upper ends of the filaments and by means of the supports 20 and 21 to which the lower ends of the filaments are attached.

The space current is led to the metal clamp 24 by means of flexible conductors 24", nickel ligaments for example, welded to said clamp and to the frame member 17, a lead-in conductor 17 also being welded to the latter. When it is desired to provide greater control of the current flowing in the central tappings to the cathodes the arrangement diagrammatically illustrated in Fig. 5 may be employed, in which instead of the central tap conductor 10 connected direct to the midpoint 12 of the choke coil 13 as illustrated in Fig. 1, it is connected to the point of connection 12 of'two similar choke coils, through two similar windings which are respectively coupled to the two choke coils 13, said windings 30 being in series with one another and connected in series between the central lead 10 and the mid-point 12. The turns of the windings 30 may be so proportioned that the current fiowin to the mid-point of the filament has any desired value relative to the space current flowing to the ends of the filament but preferably the mid-point tap carries twice the space current of either of the end taps.

Fig. 6 illustrates an arran ement in which the filament is provided wit a plurality of taps, namely three intermediate taps. In this figure the anode and external circuit are omitted. The space current conductor 6 is connected to the central tap conductor 10 to the midoint 11 of the filament and is also connecte to the midoint of a choke coil 31. The space current le through the portions lUU 31 is thus caused a) divide equally therein.

' The terminal of each of the portlons 31 is connected in turn to the mid-point of a choke coil one terminal of which is connected to an intermediate tap 33 between the central tap 11 and the lead-in wire 8 or 9 whilst the other terminals 34 of the transformers .32 are respectively connected to the leads 8 and 9. The space current flowing through each of the choke portions 31 is thus again equally divided.

Instead of e ual division of the space current as above escribed it may be caused to divide in any predetermined ratio. Preferably the inner windings of the choke coils 32 which are connected to the filament taps '33 have half as many turns as the outer winding of said transformers or chokes which are connected to the lead-in conductors 8 and 9, so that the current flowing to the taps 33 is twice as much as the current flowing to the ends of the filament.

When the filament is of zig-zag formation the space current taps may be made at the bights of the filament in the manner hereinbefore described.

Referrin to Figs. 7 and 8, the portion of the electro e structure therein illustrated is generally similar to that illustrated in Figs. 3 and 4. However, a lurality of space current taps are provide between the ends of each of the two parallel filaments in the manner now to be described.

points. The additional constraining means comprises books or eyes 35 (see Fig. 8) which project into the anode tunnels and surround the filaments. Each hook or eye 35 forms" part of a tungsten wire support 36 which is secured by welding or otherwise to a cross member 3 mounted upon the frame member 16, said member having a relatively large central perforation 38 through which the electrodes extend. Each wire support 36 passes through a perforation 39 formed in a tunnel of the anode such perforation being relatively large to avoid any possibility of the support wire 36 touching the anode. When the valve is of the type set forth in the specification of Serial No. 603,715, filed November 27th, 1922, each perforation 39 may be shielded b means of a curved member 40 which may e carried upon a nickel wire bracket 41 welded or otherwise attached to the cross member 60, the wire 36 passing through a perforation in the shield 40. In Fig. 7 two such intermediate constraining or registerin hooks are provided for each cathode ament. The cross pieces 37 for each pair of constraining hooks 35 may be separated from one another by means of insulating washers 42. The constraining hooks 35 constitute the intermediate tapping points by which the space current is led into the filament, a lead-inwire 43 (Fig. 8) being welded to each of the cross pieces 37. In this arrangement the space current is led to the intermediate tapping points through frictional contact. If desired however the tapi Sing point may be constituted by a li exible member securely attached by we ding or otherwise to the filament or a light spring of refractory material which is caused to press against the filament can be used.

current is led to the centre tap of the filament by means of inductive windings and compensating windings. The inductance 44 is connected across the filament leads 8 and 9rand an inductance 45 is connected between the central filament tap 11 and the filament lead 9. These inductances are provided respectively with two other windings 46 and 47 which are inductively coupled therewith and constitute com ensating windings. The windings 46 and 4 are connected in series For example such light spring may comprise pressing against the withone another between the filament lead 9 and the space current conductor 6. The space current thereforepasses through the windings 47 and 46, then dividing and passing through the windings 45 and 44 and the lead-in wire 9. The numbers of turns of the compensating windings and the choke coils are proportioned so that the current divides in the various taps in the desired ratio. Preferably the central filament tap carries twice as much space current as the leads 8 and 9 in which case the winding 44 has four times as many turns as the compensating winding 46 and the winding 45 has twice as many turns as the compensating winding 47.

It is preferably arranged that the filaments are tapped at approximately equally distant points and also that the total number of leadin points to the filament is odd. Further, it is preferably arranged that the intermediate space current taps carry twice the space current carried by either of the end taps.

If desired relatively small impedances such as variable inductances or resistances may be inserted in one or more of the space current taps for providing fine adjustment of the subdivision of the space current flowing into the filament.

Assumingfor example that the maximum permissible emission in an ordinary tube-is 10% of the filament current when led in at It will be understood that the invention is not limited to the articular valve arran ements diagrammatically illustrated and tie particular arrangements described for carrying it into effect, since various modifications may be made without departing from the scope of the invention.

I claim as my invention 2- 1. In a system which contains a vacuum electric tube with a filamentary cathode heated by alternating current and provided with end taps and at least one intermediate tap, the method of reducing the inequality in the distribution of heat along the filament due to the space current of the tube flowing therein which consists in leading in the space currentto the filament at its end and intermediate taps while the filament is not short-circuited as regards its heatin current and no appreciable impedance is o ered to the space current.

2. In a system which includes a vacuum electric tube with a filament cathode, a source of alternating current connected across the ends of the filament for heating the same, the filament havin end taps and at least one tap intermediate 0 its ends, a space current lead, and at least one inductive winding having an interconnection between the intermediate and end filament taps and the space current lead whereb the space current enters the filament througld all the taps and the impedance of the inductive winding is neutralized as regards the variable component of the space current but is not neutralized as regards the said source of alternating current.

3. In a system which includes a vacuum electric tube with a filament cathode, a source of alternating current connected across the ends of the filament for heating the same, the filament having end taps and at least one tap intermediate of its ends, a space current lead and at least one inductive winding having an interconnection between the intermediate and end filament taps and the space current lead whereby twice as much space current enters the intermediate ta as eachend tap and the impedance of the in uctive winding is neutralized as regards the variable component of the space current but is not neutralized as regards said source of alternating current.

4. In a system which includes a vacuum electric tube with a filament cathode, a source of alternating current connected across the ends of the filament for heating the same, the filament havin end taps and at least one ta intermediate oiits ends, a space current lea sets of magnetically coupled inductive windings having an interconnection between the intermediate and end filament taps and the space current lead whereby twice as much space current enters the intermediate ta as each end tap and the impedance of the inc uctive windings is neutralized as regards the variable com onent of the s ace current but is not neutr ized as regard said source of alternating current.

5. In a system which includes a vacuum electric tube with a filament cathode, a source of alternating current connected across the ends of the filament for heating the same, the filament having end taps and a plurality of taps intermediate of its ends, a plurality of magnetically coupled inductive windings having an interconnection betwen the several filament taps and the space current lead and through which the space current passes successively to the filament at a plurality of points whereby twice as much space current enters each intermediate tap as each and tap and the impedance of the inductive windings is neutralized as regards the variable component of the s ace current but is not neutralized as regar 5 said source of alternating current.

In testimony whereof I have hereunto subscribed my name this l6th day of June, 1924.

ERNEST YEOMAN ROBINSON. 

